Living on top of each other is more than a metaphor in these times of high-rise, high-density cities around the world. A new exhibition at Tate Modern explores just how humans do live like this, packed in in their millions, in Global Cities.

This major new show, in the Turbine Hall until August 27 2007, looks at ten urban centres – Cairo, Istanbul, Johannesburg, London, Los Angeles, Mexico City, Mumbai, Sao Paulo, Shanghai and Tokyo – through factual and artistic installations. One of the aims is indeed to provide a glimpse of things beyond the Atlantic Corridor (often the focus of anything shown in our major galleries).

The jumping off point is the statistic that 50 per cent of us now live in cities, compared with 10 per cent a century ago, and the prediction that by 2050 three-quarters of us around the world will have turned our backs on any place still deemed a town or village.

Bearing this in mind, there are obvious challenges faced by all those involved in shaping cities, as their populations expand to 10 or even 20 million. Planners and architects, say the curators, need to work with the ‘grain’ already embedded in our urban conglomerations. To give visitors a feel for this grain, five aspects of the cities’ ‘DNA’ are examined: their form, size, density, speed and diversity.

Great aerial photographs, at once beautiful and disturbingly inhuman, are plastered large on square rooms encasing video works. The works playing inside take a personal angle on the human city. There are also several new commissions on show, some with a humorous twinge.

You’ll take away a good overview of the ten cities from the exhibition. Graphic representations bring home the stunning statistics about each one, as compiled by the London School of Economics. For example, the 27 per cent of Londoners who were born abroad is shown as a large dark spot in a white circle, sat in comparison to far smaller ones in circles representing the other global cities.

London is really at the heart of things – you might perceive the whole affair as an exercise in getting tips by looking at other cities on what to do and what not to do as the capital builds out into the Thames Gateway in the coming years. But there really is very little prescriptive interpretation with the installations, though it would inject some humour into development if London’s movers and shakers took notice of Nigel Coates’ Mixtacity model.

The fantasy work shows the area east of Canary Wharf as it might be built up if driven by artistic spirit instead of boring things like economics. It begins with the famous twin office towers being composed of Bourbon biscuits, moving on to a bridge with ‘giantess hands’ at either end.

Eco-evangelists would take over the ‘Smoking Gun Towers’, from where they can spot carbon emission violations and then there’s Buddha Boulevard, a ‘live/love/work district that began with direct links to Delhi’. Personally, I might take up residence in one of the buildings made of humbugs.

If being in city crowds makes your hackles rise, you’ll like the illusion created by Yang Zhenzhong’s video installation, Let’s Puff. A girl in one video blows hard, as hard as the wolf in Little Red Riding Hood, and as she huffs and puffs, Nanjing Road in Shanghai – one of the main commercial streets – is blown backwards, away from her.

And if you get that sardine feeling on the tube, spare a thought for those living in Mumbai and Cairo. Mountain-like contoured wooden structures are used to show the density of different districts in London, Cairo, Mexico City, and Mumbai. London and Mexico look like tame little spread-out ranges compared to the steeples of Cairo and Mumbai.

The fact that the latter two are stabilised by taut cables is symbolic, but high-density can actually be more sustainable; sprawling metropolises like Los Angeles dictate the use of that environmentally unfriendly form of transport, the car.

It’s also interesting to note that some of the peaks of London are far more desirable areas, like Notting Hill, than low-density areas (the City has few actual residents, for example). High-density needn’t mean unpopular – almost a tautology.

In fact, touching aspects of how people colonise cities are highlighted. Japanese revellers after the success of their football team in a 2002 World Cup match are shown taking over the streets of Tokyo; bridges over the Nile are used for evening picnics in Cairo.

Another thing our capital city has to be thankful for is its wealth of green spaces. In a video by Osman Bozkurt, families in Istanbul are captured picnicking in one of the only wide open green spaces available to them, slap bang in the middle of a booming spaghetti junction. When interviewed, they explain that they are risking life and limb to spend time in this noisy, polluted, ‘oasis’ – last week, a father says, a boy was killed crossing the road to get there.

Equally, the wobbly Millennium Bridge is nothing compared to the dangerous wave-lapped sea wall that pilgrims cross from Mumbai to the Haji Ali Mosque, shown in Eva Koch’s video, NoMad.

Lastly, have a look at Fritz Haeg’s installation upstairs, Edible Estates regional prototype garden #4, London UK, to see how you might begin about transforming an under-used urban green space near you into a vegetable plot.